Infrared imagers have many possible applications and can be used for vehicle collision avoidance, perimeter surveillance, engine diagnosis and monitoring, missile seeking, and large-area imagers. The disclosed dual-band infrared photo-detector array and imager can operate at much higher temperature while still achieving high sensitivity. The higher operating temperature (>150-200K) made possible by the disclosed invention could greatly reduce the size and cost of the infrared imagers since cryogenic cooling would not be needed. The higher temperature operation could reduce the cost of platforms containing such imagers and could improve the reliability of their infrared sensors. The possibility of operating at temperatures achievable with thermoelectric-coolers increases the breadth of commercial applications since the imagers and detector arrays would be more affordable.
Most prior detectors of infrared imagers have pixels that are defined by etching mesa structures that contain the light absorbing layer. FIG. 1a shows the cross-sectional structure and FIG. 1b shows a perspective view of a prior dual-band detector in which the mesa structure is formed by etching through the absorbing layer for one wavelength band but not etching the mesa through the absorbing structure for a second wavelength band. See M. Vuillermet, F. Pistone and Y. Reibel, “Latest developments in MCT for next generation of infrared staring arrays,” Proceedings SPIE, Vol. 7481, 2009, p. 74810F. In each pixel of this detector, the detecting regions for the two bands are slightly offset from each other. Also, each diode detector of a given pixel has a separate electrical contact, thereby allowing true simultaneous detection of light in the two wavelength bands. There is a layer comprising wider bandgap material (the NIR barrier) located between the two light-absorbing layers that prevents electrical short-circuiting between the two diodes. This barrier blocks both the electrons and the holes. In general, the light-absorbing layer that absorbs the shorter-wavelength light is located closer to the incident light, which is the layer located closer to the substrate in the case shown in the figure.
In the pixels of other prior detectors, the light absorbing regions for the two wavelength bands are stacked directly above each other and both of these light-absorbing layers are located within the same mesa. See G. Destefanis, et al., “Bi-color and dual-band HgCdTe infrared focal plane arrays at DEFIR,” Proceedings SPIE Vol. 6206, 2006, p. 62060R. This structure is illustrated in FIGS. 2a and 2b. Each pixel consists of two back-to-back N-on-P photodiodes. Each pixel has one independent contact and also has a contact that is common to the other pixels of the array. The two photodiodes of a pixel are electrically contacted and biased through the single contact located on the top of the mesa and through the common contact located on the substrate side of the structure. By switching the bias voltage for a pixel from a positive value to a negative value, it is possible to extract the photodetected signal from one or the other P/N junction photodiode.
We recently invented a single band detector (albeit having very wide bandwidth) that has a reduce volume of its light-absorbing material. This detector contains multiple pyramid shaped features formed in each pixel, with those pyramids located on the side of the detector facing the incident light. See U.S. patent application Ser. No. 12/554,788 filed Aug. 20, 2009 and FIG. 6 thereof. The pyramids are etched into the light absorbing layer. The pyramids are etched only partly through the light-absorbing layer so that there remains a physically continuous base region of the light-absorbing layer to permit the majority carriers to be conducted to electrical contacts formed at the edges of the detector array. This detector also contains mesas etched through the heavily doped collector or extractor layer of the P/N diode. The mesas are not formed in the main light-absorbing layer and the mesas face away from the incident light. The pixel-specific electrical contact for a given pixel is formed onto these collector mesas. The spatial extent of a given pixel is defined by the electrical contact made to these mesas. There can be more than one mesa for each pixel and there are multiple pyramids in each pixel. As a variation of this detector structure, the mesas can be made to have the shape of shallow pyramids. Those mesas also can be coated with a metal reflector that conforms to the shape of the mesas.
In our prior detector of FIG. 3 herein, the light is incident from the side containing the pyramids rather than from the substrate side of the detector. Thus, this detector is unlike the prior detectors illustrated in FIGS. 1 and 2. But a potential weakness of our prior detector and detector array is that its substrate must be removed in order to fabricate its mesas and electrical contacts and to form the electrical connections between that detector array and the electronic readout circuit. Thus the resultant thin detector-array structure of FIG. 3 is more difficult to fabricate and to integrate with the readout circuit.
For the detectors of FIGS. 1a, 1b and 2, the pixel for the longer-wavelength band is defined by the single mesa of that pixel. For the detector of FIG. 3, the pixel for the single band is defined by the extent of the electrical contact made to the one or more collector mesas comprising a pixel. The electrical contact made to the light-absorbing layer is the common contact of the detector array. The pyramid-shaped regions of the light-absorbing layer do not specifically define the extent of a pixel but rather they extend throughout the light-facing surface of the array.
Besides our prior detector of FIG. 3, there are no other infrared detector structures to our knowledge that have pyramids or other such geometric shapes formed on both opposing sides of the detector. There are, however, prior solar cells that have pyramids or other surface texture etched into the substrate, with that textured surface facing the incident light (see FIG. 4). See A. Sinton, et al., “27.5 percent silicon concentrator solar cells,” IEEE Electron Device Letters, Vol. EDL-7, no. 10, October 1986, p. 567. The textured surface reduces the front-surface reflection of the incident light and improves the coupling of that incident light into the solar cell. Since the substrate material of the solar cell absorbs the incident light, their pyramids for reducing the front-surface reflection of incident light are etched partially through the light-absorbing material of the solar cell.
There also are prior solar cells that have geometrically shaped features formed both on the side facing the incident light as well as on the side facing away from the incident light. See R. Brendel, et al., “Ultrathin crystalline silicon solar cells on glass substrates,” Applied Physics Letters, Vol. 70, no. 3, 20 Jan. 1997, p. 390. These geometric features are formed into both sides of a substrate such as glass, which is transparent to the light to be absorbed by the solar cell. Then a laterally continuous, or contiguous, film of light-absorbing material is formed onto one shaped side of the substrate (see FIG. 5). Solar cells that have pyramids or other geometrical shapes formed on both sides of the substrate were analyzed and found to have improved trapping and absorption of the incident light. See Campbell and M. A. Green, “Light trapping properties of pyramidically textured surfaces,” Journal of Applied Physics, Vol. 62, no. 1, July 1987, p. 243. For the solar cells described by Brendel et al., one set or both sets of electrical contacts can be formed on the side of the solar cell that faces away from the incident light. That side of the solar cell is covered with flat metal reflectors that also serve as electrical conductors to those electrical contacts.
In other prior solar cells, geometric shapes such as pyramids are formed into one side of a first slab of transparent conducting oxide material. One or two layers of thin, but laterally continuous, light-absorbing materials are formed above the shaped surface of the transparent conducting oxide. See P. Obermeyer, et al., “Advanced light trapping management by diffractive interlayer for thin-film silicon solar cells,” Applied Physics Letters, Vol. 92, p. 181102 (2008) and C. Haase and H. Stiebig, “Thin-film silicon solar cells with efficient periodic light trapping texture,” Applied Physics Letters, Vol. 91, p. 061116 (2007). The side of the contiguous light-absorbing layers opposite that first transparent conducting oxide slab also has a pyramidal shape. That opposite side of the top-most absorbing layer is covered with a second slab of transparent conducting oxide. This second slab of transparent conducting oxide has a shaped surface facing the light-absorbing layers and a flat surface facing away from the light-absorbing layers. The flat surface of the second slab of transparent conducting oxide then can be covered with a metal reflector. For these solar cells, the electrical contacts are formed on both opposing sides of the light-absorbing layers with electrical connections provided by means of the transparent conducting oxide material.
Many high-sensitivity focal-plane photo-detector arrays for detecting light at mid-wave infrared (MWIR) wavelengths or longer need to be cooled to cryogenic temperatures (e.g., 77K and lower) in order to sufficiently reduce their internally produced noise current to levels that are below the background noise of the scene. However, cryogenic coolers, such a Stirling coolers, are bulky and they involve moving parts that can reduce the reliability of the overall system. If the operating temperature of the detector array can be increased to 200° K and higher, it approaches the range of temperatures that can be attained by thermoelectric (TE) coolers that do not involve moving parts. If the operating temperature can be increased even to 150-200° K, it can be cooled by radiative means for imagers used in space. Thus, there is a need for infrared detector arrays that can operate with low noise current at temperatures of 150° K and higher.
The noise current of an un-illuminated infrared detector, or its dark current, has several major components. One component is a generation/recombination current (G/R current) that is limited by G/R centers at material interfaces such a homojunctions or heterojunctions in the detector. Another component is a diffusion current that, for high quality materials, is limited by thermal generation in the bulk of the light-absorbing material. Yet another component is a surface-recombination current due to interface electronic states resulting from un-passivated dangling chemical-bonds at the outer boundaries of the detector semiconductor material. For many common infrared detector materials, such as HgCdTe and antimony-based compounds, the G/R current typically dominates the dark current at low temperatures, such as below 120-150K. However, at higher temperatures, the diffusion current and the thermal generation current within the bulk absorber regions dominate the dark current.
One way to reduce the ratio of diffusion current to G/R current at the higher operating temperatures is to reduce the volume of the absorber material. However, this reduction of absorber volume typically also results in a reduction of the photon absorption efficiency or quantum efficiency of the infrared detector. The disclosed detector achieves both reduced diffusion current as well as high quantum efficiency to permit operation at higher temperatures. The reduced diffusion current is accomplished by reducing the volume of absorber material, for a given input cross-sectional area of detector array or, alternatively, a given pixel area. The high quantum efficiency is achieved by using geometrical features that greatly reduce the net front-side reflection of the incident light and also that trap the incident light such that the light makes multiple passes through the absorber regions.
FIG. 6 shows results from calculations of the absorbance (normalized absorption) of a detector comprising a uniform thickness of MWIR (MidWave Infra Red) absorbing material for various wavelengths of incident light. Such a detector structure is representative of the mesa structures of FIGS. 1 and 2 as well as the planar structure for the shorter-wavelength detecting portion of FIG. 1. These calculations assumed an InSb light-absorbing material but the disclosed detector array actually could comprise any light-absorbing material that absorbs light at the desired range of wavelengths for its two detection bands. The results indicate that the thickness of the absorbing layer should be at least twice the wavelength of the incident light in order to achieve maximum broadband absorbance. For thinner absorbing layers, there are strong oscillations in the dependence of absorbance on layer thickness that can be associated with multi-pass optical cavity effects (Fabry-Perot cavity resonances). Since it is desirable to reduce the volume of absorber material, a typical prior detector could have a metal reflector located at the side of the detector opposite the incident light. This metal reflector has greater reflectance that the reflection associated with just the high refractive index of the light-absorbing material, thereby improving the detector quantum efficiency, especially at the wavelengths of the Fabry Perot resonances.
The results in FIG. 6 also show that the absorbance obtained for very thick layers of absorber approaches a value of approximately 0.6. This low absorbance is due to the front-surface reflection of the light, since the refractive index of the absorber material is much larger than 1. Many prior detectors have anti-reflection (AR) coatings that comprise one or more layers of dielectric films that have the desired combination of film thickness and refractive index to minimize the reflection at specific wavelengths of the incident light. However, it is difficult to obtain AR coatings that are suitable for a wide range of incident wavelengths, such as a range approaching one octave. Also, it is difficult to obtain AR coatings for dual-band detection unless the wavelengths of those two bands are multiples of each other. Thus, it would be very difficult to obtain an AR coating that provides low reflection at 0.9-1.6 micron wavelength as well as at 3.0-5.0 micron wavelength, for example. There remains a need for a multi-band infrared detector that has low front-side reflection as well as high external quantum efficiency at all detected bands.